at idle, my 10yr old mercedes C180 sometimes revs high for no reason. It will idle at about 2000rpm but when I touch the acclerator, it goes back to normal idle speed of about 800rpm. Why is this happeneing? has anyone else had a similar experience?

Sounds like a problem with the automatic choke,
Common fault on a lot of cars, get a manual conversion kit, should sort it out.

Is it when you first start up your engine? if so its because its warming up its the automatic choke

if you have a friend mechanic that is familiar with such cars ask him for a few calibration or adjustment.. this will work.. sometimes just as you have experienced with your car, i have also similar problem, then my friend diagnosed it very well.. no sweat... it worked well.. free of charge

I think I have the same car as you and I had the same problem - the revs were very high when the engine was just switched on (please excuse my lack of technical knowledge!). I'm afraid you'll need a trip to the garage - my hubby took mine and they fixed it. Good luck!